1. Field of the Invention
The present invention relates to sensor, and more particularly to a flow rate sensor.
2. Description of Related Art
A conventional mechanical flow rate sensor in accordance with the prior art shown in FIG. 7 comprises tube (30) and a propeller (31) rotatably mounted in the tube (30). The propeller (31) peripherally abuts an inner periphery of the tube (30) such that the propeller (31) is rotated when the fluid passing through the propeller (31) in the tube (30).
A magnetic sensor (301) is secured on an outer periphery of the tube (30). The propeller (31) includes multiple blades (not numbered) and a magnet (310) is securely in a distal end of a corresponding one of the multiple blades of the propeller (31). The magnetic sensor (301) measures the rotation rate of the propeller (31) by sensing the rotation force of the magnet (310) during turning around to calculate the flow rate in the tube (30).
However, the dirt in the fluid may be adhered on the blades of the propeller (31) after being used for a period of time and the propeller would not be smoothly operated. Consequently, the accuracy of the conventional flow rate sensor is debased. 
For solving the above problem, a conventional digital flow rate sensor, as shown in FIG. 8, is marketed. The conventional digital flow rate sensor includes a body (40) having a passage (401) linearly defined in and extending through the body (40). A recess (404) is defined in a top portion of the body (40). A first path (402) is defined in the body (40) and communicates with the passage (401) and a first end of the recess (404), and a second path (403) is defined in the body (40) and communicates with the passage (401) and a second end of the recess (404). A cover (405) is securely attached to the top portion of the body (40) by radiofrequency welding for closing the recess (404). A chip (406) is secured on a bottom of the recess (404) and corresponds to the first path (402) in the body (40) for calculating a different of a pressure in the passage (401) via the second path (403) and the first path (402). Multiple leads (408) are embedded in a bottom of the recess is (404) and electrically connected to the chip (406). Each lead (408) extending through the cover (405) and electrically connected to a circuit board (409). The chip (406) and the leads (408) in the recess (404) are packaged by porosity glue (407).
However, the chip (406) may be wetted and causes a leakage of electricity when the fluid flows through the passage and the pressure in the recess (404) is continually raised. The porosity glue (407) may contain the fluid from the recess (404) and cause a short circuit on the chip (406). In addition, the cover (405) is welded to the body (10) for  closing the recess in the body (10) by radiofrequency welding. However, the radiofrequency welding cannot provide a completely airtight effect between the body (40) and the cover (405).
The present invention has arisen to mitigate and/or obviate the disadvantages of the conventional flow rate sensors.